1. Field of the Invention
The present invention relates to wireless communication, and in particular, relates to wireless communication in a data network having a large number of mobile terminal units communicating with each other through one or more cellularized base stations, which are connected by wired or wireless links to form a back-haul data network.
2. Discussion of the Related Art
One-way messaging or paging application is a well-established economical technology for transmitting short messages to a mobile pager. In one-way paging, typically, a caller calls a paging station using a telephone number assigned to the pager for which the message is intended. The message is then provided to the service computer at the paging station which, in turn, broadcasts the message, using high power transmitters, in its service area. In the broadcast message, the user""s message is packaged in a data frame which contains an address code which identifies the recipient pager. The recipient pager is then activated by the receipt of the message, causing a vibration or an audible signal, thereby informing the callee the arrival of the message. Other applications have also been developed for one-way paging. For example, the service computer may broadcast to subscribing users stock quotes, weather information, results of sports events, and other information of interests.
However, one-way paging is limited in that the callee must respond to the caller through an alternative network. Typically, the alternative network is the telephone network. However, because users of paging systems are by nature mobile, to respond to a page through the telephone network requires easy access to the telephone network. Access to the telephone network is sometimes inconvenient. This inconvenience can be eliminated if a two-way communication capability is provided to the pager.
Other applications using one-way messaging technology have also been developed. Some of these applications relate to dissemination of information, such as weather reports, stock quotes, news headlines, or results of sports events. However, because communication is one-way in these applications, the amount or type of information that can be disseminated in this manner is constrained by the recipient""s inability to select in real time information he or she desires. A two-way communication capability can provide xe2x80x9cinformation-on-demandxe2x80x9d services which are more suitable to individual user needs, as well as providing a more efficient use of the broadcast spectrum.
Two-way communication services are, however, expensive because of complexity. For example, in a cellular digital packet data (CDPD) system, a large number of cellularized base stations are distributed all over the service area. Cellularizing the service area offers two advantages: (i) allow mobile units and base stations to transmit at higher data rate with relatively low power, since the expected distance between a mobile unit and a base station in the vicinity is short; and (ii) larger capacity is provided because base stations which are separated by large enough distances can use the same radio channels. Such a system provides thus very high capacity, low response delay and allows the mobile units to transmit at relatively high data rates. In such a system, two-way symmetrical and reliable data links can also be provided.
In a CDPD system, because connectivity is maintained over the entire duration of a data communication session, multiple channels must be provided to allow multiple sessions to be maintained simultaneously. To locate a recipient mobile unit of a message, the network broadcasts the address of the recipient mobile unit from all the base stations in the service area until the recipient unit responds. Thus, a large amount of network resources is dedicated to locating mobile units. Further, to maintain continuous connectivity and to allow real time performance, when the session is established, the CDPD unit is associated with a base station with which it communicates. In addition, because a mobile unit can be expected to be used in a moving vehicle, it is possible that the mobile unit moves out of the service range of the initially associated base station and moves into the service range or ranges of one or more such base stations during the duration of a session. Thus, provisions must be made to disengage an associated base station and to engage an additional base station or stations (xe2x80x9chand offxe2x80x9d) during the course of the session. The control mechanisms for maintaining a CDPD session, including tasks typically termed xe2x80x9cconnectivity managementxe2x80x9d and xe2x80x9cmobility managementxe2x80x9d, involve sophisticated algorithms which require high performance computers to handle setting up the session, maintaining the session, and tracking the communicating mobile units as they move between service areas of the cellularized base stations. The complexity of the system requires a large investment in expensive equipment. Often, these control mechanisms are centralized, i.e. a large network switching or control center is provided to handle the mobile units in a given service area, so that, at times of heavy data traffic, the network control center may become a bottle neck, introducing undesirable latency into the system.
Another major disadvantage of the CDPD system is the requirement that the receiver of the mobile unit must be on at all times to receive messages. As a result, such a mobile unit requires a battery that is, at the present time, too undesirably bulky for mobile use. It would also be extremely difficult for power-saving features to be provided in such a mobile unit. Unlike a pager, which monitors only one paging channel, the mobile unit of a CDPD system must monitor a different radio channel when it is located in a different cell. Further, control information (e.g. timing information and recipient addresses) are broadcast by base stations in dedicated control channels. While a pager can be assigned a periodically occurring time-slot during which it xe2x80x9cwakesxe2x80x9d up to check for messages, a similar scheme in a mobile unit of a CDPD system would require timing in all the cellular base stations to be synchronized. Failure to synchronize all base station can lead to a mobile unit checking for the broadcast of its address during the wrong time slot.
Thus, a data network which allows bidirectional communication between mobile units without sophisticated equipment to perform connectivity and mobility management tasks is highly desirable.
The present invention provides for a two-way personal mobile terminal having a first receiver to receive broadcast messages from a high-power transmitter and a transceiver to transmit and receive control and data signals between it and a local cellularized base station. In one embodiment, the high-power transmitter is a broadcast transmitter compatible with one-way paging systems. In fact, in that embodiment, the high-power transmitter broadcasts a data message intended for a two-way personal mobile terminal, as if the data message is intended for a one-way pager. In that embodiment, the personal mobile terminal receives its message through its first receiver. The receiver demodulates the broadcast signal, and passes the demodulated signal to an on-board microprocessor to be decoded and displayed on a display of the personal mobile terminal, just like a one-way pager. However, under the present invention, the user of the personal mobile terminal may respond by transmitting a reply message using the personal mobile terminal""s transceiver. This reply message is received by a cellularized local base station, which then relays the message to the network control center for further handling.
Thus, the present invention provides two-way communication capability by leveraging the existing infrastructure of one-way paging. For example, expensive resources such as high-power paging transmitters can be used for both one-way and two-way applications. Further, the personal mobile terminal can be the basis for a data network, which can be built by providing relatively inexpensive (i.e. relative to high cost high-power transmitters) cellularized base stations. The existing one-way paging user-base can converted to become users of this data network by simply purchasing such a personal mobile terminal. This data network provides additional applications beyond mere two-way paging.
Because the personal mobile terminal is receive-only with respect to messages from one-way paging towers, and transmits messages only to the cellularized base stations within a very limited range of its immediate vicinity, the power consumption for such personal mobile terminal is small. Thus, the personal mobile terminal can be made as compact as a one-way pager.